I sell primarily books and ship them Media Mail, unless they are expedited. I was advised by my local Post Office that even the brief message at the bottom of the Amazon packing slip:

+Thanks for buying on Amazon Marketplace. To provide feedback for the seller please visit xxxxxxxxxxx. To contact the seller, go to Your Orders in Your Account. Click the seller’s name under the appropriate product. Then, in the “Further Information” section, click “Contact the Seller.”+

constitutes a “personal message” and changes the entire package into Parcel Post. They stated that if their staff opens the package for inspection and finds the packing slip with that message, the recipient (my customer) will receive the package “Postage Due.”

Have any other book sellers experienced this problem. Have you had packages delivered “Postage Due” to customers because of the Amazon packing slip, or because of thank-you messages on your invoices?

Incidental First-Class Mail matter may be enclosed in or attached to any Media Mail or Library Mail piece without payment of First-Class Mail postage. An incidental First-Class Mail attachment or enclosure must be matter that, if mailed separately, would require First-Class Mail postage, is closely associated with but secondary to the host piece, and is prepared so as not to interfere with postal processing. An incidental First-Class Mail attachment or enclosure may be a bill for the product or publication, a statement of account for past products or publications, or a personal message or greeting included with a product, publication, or parcel. Postage at the Media Mail or Library Mail price for the host piece is based on the combined weight of the host piece and the incidental First-Class Mail attachment or enclosure.

You either have complete newbies or ill-advised clerks at that post office. If your inclosure is the amazon packing slip, that is as Quirky indicated simply “incidental”. Besides, even if you enclosed a hand written letter/note or greeting card, and your package did indeed meet media guidelines, the worst they could do is charge you the " first class enclosure fee" which is 49cents. They could NOT change your mail class based on that invoice/note. Contact consumer affairs for your area and file complaint.

We sell some books, but also music and DVDs which qualify for media mail as well. We have sent many media mail packages with this on the packing slip and not once had any issues with it being returned or marked as postage due to the buyer.

Technically, if the inspectors were to open your media mail package, they could violate the package and mark it as postage due, but in my experience and interaction with fellow sellers, none have experienced this.

We are aware of this policy and when prepping the packing slip, we remove this portion of it just to be on the safe side.